Wearing surface for tires for winter conditions

ABSTRACT

A wearing surface for tires includes a rubber or rubber mix and an admixture of friction-improving materials. The admixture of friction-improving materials include a mixture of crystalline chrysolite (olivine) and a metal powder. The metal powder can be steel or an aluminum alloy.

BACKGROUND OF THE INVENTION

The invention under consideration is a composite material for use aswearing surface on rubber tires for winter conditions.

There are several factors influencing the grip of a car tire on ice andsnow, such as the tire design, the non skid pattern as well as thematerial composition for the wearing surface.

During the past years a lot of research work and money have been spenton developing tires with improved performance, especially for winterdriving under icy and slippery road conditions. Especially due to theheavy wear of pavement material experienced with the use of cogged tiresand resulting environmental problems, efforts have been made to developtires for improved performance without using cogs. A lot of designchanges in the non skid patterns have been tried, as well as differencerubber mixtures for the wearing surface with or without so calledfriction increasing elements like grained or particle formed materialsembedded.

Examples of the present position of developments may be found bystudying the following patent publications: U.S. Pat. No. 2,582,701,U.S. Pat. No. 2,672,910, U.S. Pat. No. 2,675,047, U.S. Pat. No.2,766,800, U.S. Pat. No. 3,062,255, GB 1578077, EP 442155, FR 2317109,WO 85/05329, DE 2101224, DE 3220603, U.S. Pat. No. 1,578,121, as well asNO applications 903245 and 9033259.

The applicant has performed a number of practical tests coveringdifferent rubber mixtures as well as variable proportions of rubber andparticles in the wearing surface. Through this work, the applicant hasreached the conclusion that parameters like:

range of particle hardness

particle size and distribution

percentage of particles mixed into the wearing surface material

are essential for the friction of the tire against icy and slippery roadsurfaces. Depending on the hardness level of the material to be mixedin, the applicant has found that the percentage of particles mixed intothe wearing surface material may be considerably increased by reducingthe grain size.

SUMMARY OF THE INVENTION

Generally speaking, the frictional material for mixing into a wearingsurface rubber compound is according to the present inventioncharacterized by a mixture of crystalline chrysolite (olivine) and ametal powder, being for example a steel powder and/or a powder from analuminium alloy, and if desired an additional friction supportingmaterial having good binding force to the rubber mixture.

A proposed admixture for the frictional material is 20 to 60% by weight,preferably approximately 45% by weight of the total wearing surfacecompound.

Regarding composition of the frictional material it is suggested, afterperforming experiments, that the best results are obtained usingapproximate values of powder materials in the order of 5-20% steel,30-60% crystalline olivine and additionally, if desired, some powdermaterial.

Regarding particle size it is concluded, after performing experiments,that the materials be finely ground to the extent possible. The olivinepowder as well as the other materials used should as an example maintaina particle size of 0.001 to 0.1 mm, the greater part of the particlesbeing preferably in the area 0.0015 to 0.022 mm.

By using a mixture as mentioned above, a particularly stabilizedrelation between rubber- and friction material will occur due to thenature of the compound and the grading as well.

As the admixture of additional material, this part may as an exampleconsist of coke powder. This coke powder may also partly or as a wholebe exchanged by a silicon carbide in powder form, for example, replacingfrom 3-40% of the coke powder. The use of coke powder in the frictionmaterial offers particularly good frictional properties, but the cokepowder by itself reduces the wear resistance of the rubber compound. Theolivine powder has proved to give a wearing surface with very goodfrictional properties in addition to the fact that the particle shape ispositive for the binding to rubber. It has further been found possibleto achieve a very good and homogenous mixture of olivine and steelpowder, and the steel has been seen during the vulcanisation process toincrease the heat conducting properties of the mixture quiteconsiderably, leading to a better and smoother vulcanization of thetread to the cord. The steel powder also assists in improving wearresistance and steadiness of the tread.

The raw materials of the friction compound are initially mixed in drycondition in a drum mixer or similar for approximately 10 minutes.During this dry mixing process, the olivine powder has demonstratedespecially good qualities since it assists in removing possible oxidesor other surface coatings from the steel particles. This is importantbecause any kind of coating on the steel particles will reduce the finalbinding to the rubber mixture. Subsequently a socalled "primer", beingin itself of a known type and make, is added, and the stirring iscontinued for for instance 30 minutes until the primer, originallyhaving a liquid consistence, has been completely mixed with the solidmaterials and has dried. The treatment with primer is preferably done intwo steps with a drying period in between. Primers well suited for thepurpose are marketed under the product names Chemosil 211 and Chemosil220. After the friction compound is readily coated with primer which hasbeen allowed to dry completely, it is mixed into the ordinary wearingsurface rubber of the tire.

A preferred way of producing wearing surfaces is according to thisinvention to feed the friction compound into a mixing machine where itis mixed with the rubber. The final mixture is delivered from the mixingmachine in pieces or strips of dimensions suitable for feeding anordinary extruder where the compound is further extruded to a stripsuitable for forming the tread on the tire. After a strip of thicknesssuitable to form the tread has been applied on the tire, the tread jointis rubbed down using a rowel, and the tire is then shifted to apressurized and heated vulcanizing form in order to press the treadpattern and vulcanize the tread to the tire in an ordinary and wellknown manner.

Concerning the steel materials used, these may very well be stainless.Provided the cost of stainless steel is considered being on the higherside, some other high grade steel may as well be considered used,examples may be steel powder qualities marketed under the product namesFLEXGRAIN or FERROGENT. Steel powder may also be substituted partly ortotally by a granulated or powdered light metal alloy like for instancealuminium alloy AlMg 5 (NS 17552).

The admixture of friction material into the rubber mix may be performedin different ways. At the rubber manufacturer, the friction material mayfor instance be mixed into the rubber while the latter is still flowing.Alternatively the friction material may be rolled into the rubbermixture while the latter is more or less flowing. As mentioned earlier,friction material as much as 20-60% by weight of the finished rubbermixture into which it is going to be admixed, should be used, preferablythe percentage should be around 45.

An important feature of the invention is that the steel powder used isof a grain size similar to the remaining materials confirming thefriction compound. Preferably the proportions by weight of the mixtureshould be 1 part steel powder to 7 parts of the remaining frictioncompound. This reads a single steel particle may be surrounded by 7other particles from the friction compound, leading to an improvedvulcanization of the entire friction compound into the rubber mix sincethe heat conduction of steel is better than for the other materials. Therubber is thereby more or less melted onto the particles of the frictionmaterial during vulcanization.

Experiments have shown that by using metal powder in the form of analuminium alloy, the heat conduction from the tire is improved. Lessproblems with the tire running hot and also reduced wear are thereforeexpected as a result.

Regarding tread patterns of current interest, the applicant has arrivedat the conclusion that, when using the invented compound for the wearingsurface, the tread pattern may with advantage be designed with lessnumber of grooves, diagonally as well as radially, so leading to anincreased contact area between tire and rad surface. This is due to thenew compound increasing the adhesion against ice as compared to what hasbeen the fact for tires till now. Through experiments, the applicant hasreached the conclusion that a specially favourable tread pattern mayconsist of substantially square pieces of material having a flat contactsurface against the road, and being dissociated by grooves at rightangles to each other in such a way that a set of parallel grooves arerunning in the longitudinal direction (diagonal grooves), whereasanother set are running in the transversal direction of the tire wearingsurface (radial grooves).

The groove width should be waning towards the bottom of the groove, forinstance 7 mm width at the top decreasing to approximately 5 mm at thebottom. By this, occurrences like slash, snow or ice choking the groovesmay be avoided.

A tread pattern preferred by the inventor comprises square blocks havingthe approximate size of 20×20 mm, separated by grooves of approximately5 mm width for fast drainage of water.

Car tires having a wearing surface compound consisting of the presentinvention, may be manufactured as ordinary new tires or by retreading ofsecond hand tires.

BRIEF DESCRIPTION OF THE DRAWINGS

The manufacturing of a car tire by retreading using the present inventedcompound, is illustrated by the enclosed figures where:

FIG. 1 displays the cross-section of a car tire having a wearing surfaceapplied in layers using a compound similar to the present invention,

FIG. 2 shows a magnified sector from the FIG. 1 tire, whereas

FIG. 3 shows a sector from a tire as described by the inventor, as seentowards the wearing surface from above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an ordinary method used for retreading of tires,coiling of the desired number of layers--in this case 5--of an endlessstrip of wearing surface material 2. When manufacturing tires accordingto the present invention, a wearing surface material may be used thathas got the desired admixture of grained or powder material already inthe strip production phase.

As mentioned earlier, the new wearing surface is vulcanized to the tirein a vulcanizing form at a temperature of typically 150°-165° C. and ata pressure of 7-25 bars. Samples cut from tires retreaded by applyingmultiple layers of thin strip show, according to the inventor, that thefriction compound has been distributed to neighbouring layers of thewearing surface in such a way that the entire wearing surface has got analmost homogenous material structure, schematically illustrated also inFIG. 2. It is appropriate, therefore, to squeeze out the tread patternin the usual and well known manner during vulcanization withoutdestroying the compound in any way.

As mentioned before, experiments have shown that tires containing theinvented compound in the wearing surface may as well have a relativelybigger pattern contact area. A preferred tread pattern design asproposed by the inventor, is a checked one having the square form 6shown in FIG. 3 with longitudinal and transversal intermediate groovesfor the purpose of good water drainage. Strip- and belt formed wearingsurface rubber for use during retreading of tires are usually suppliedin different thicknesses ranging from 2 to 3-4 mm. A common thicknessrange is 3-4 mm. A good mix of the wearing surface compound has beenobtained when applied in layers down to a thickness of 2-4 mm.

Having built up a sufficient thickness of the wearing surface usingstrips of belt, the tire wearing surface is smoothened and the tire isshifted to a vulcanizing form for vulcanization.

I claim:
 1. Wearing surface for tires, comprising:a rubber or rubbermix; and an admixture of friction improving materials, characterized bythe friction improving materials constituting approximately 20-60% byweight of the rubber mix, the admixture comprising a mixture ofcrystalline chrysolite (olivine) powder and a metal powder, the metalpowder being selected from the group comprising a steel powder and/or apowder from an aluminum alloy.
 2. Wearing surface for tires as stated inclaim 1, wherein:the friction improving materials comprise approximately5-20% powdered or granulated steel material, and approximately 30-60%crystalline chrysolite powder.
 3. Wearing surface for tires as stated inclaim 2 wherein the steel material consists of stainless steel. 4.Wearing surface for tires as stated in claim 1, wherein:the metal powderin its entirety consists of a powder from an aluminum alloy.
 5. Wearingsurface for tires as stated in claim 1, wherein:the chrysolite powderhas a particle size in the range 0.001-0.1 mm.
 6. Wearing surface fortires as stated in claim 5, wherein:the major portion of the frictionimproving materials have a particle size in the range of 0.0015-0.022mm.
 7. Wearing surface for tires as stated in claim 5, wherein:the majorportion of the chrysolite has a particle size in the range of0.0015-0.022 mm.
 8. Wearing surface for tires as stated in claim 1,wherein:the friction improving materials further comprise an additionalfriction supporting material having a good binding force to the rubberor rubber mixture.
 9. Wearing surface for tires as stated in claim 8,wherein:the additional friction supporting material comprises cokepowder.
 10. Wearing surface for tires as stated in claim 1, wherein:themetal powder comprises stainless steel.
 11. Wearing surface for tires asstated in claim 9, wherein:the additional friction supporting compoundfurther comprises silicon carbide.
 12. Wearing surface for tires asstated in claim 11, wherein:the silicon carbide replaces 3-40% of thecoke powder.
 13. Wearing surface for tires as stated in claim 1,wherein:the friction improving materials have a particle size in therange 0.001-0.1 mm.